The present invention relates to semiconductor devices and methods for manufacturing the same. The present invention also relates to photodiode devices and methods for manufacturing the same. The present invention additionally relates to high-speed, long-wavelength photodiode devices and improved methods for manufacturing the same. The present invention further relates to techniques for growing semiconductor photodiode devices on substrates.
With the rapid development of lightwave communications, low-cost, high-performance optical photodiodes are required for a variety of system applications. Photodiodes are p-n junction semiconductor devices that produce a significant photocurrent when illuminated. Photodiodes operate based on principals of photoconductivity, which is an enhancement of the conductivity of p-n semiconductor junctions due to the absorption of electromagnetic radiation. Photoconductivity may result from the action of radiation in the visible portion of the spectrum in some materials.
There are two main classes of photodiodes currently in use today. These are depletion-layer diodes and avalanche diodes. Depletion-layer photodiodes are based on a reverse-biased p-n junction configuration operated below a particular breakdown voltage. When exposed to electromagnetic radiation of a desired wavelength, excess charge carriers may be produced as a result of absorption. Such carriers may be formed as electron-hole pairs. Those electron-hole pairs generated in or near to a depletion layer at a junction, cross the junction and produce a photocurrent. Avalanche diodes are based on reverse-biased p-n junction diodes operated at voltages above the breakdown voltage. Current multiplication of electron-hole pairs, generated by incident electromagnetic radiation, occurs due to the xe2x80x9cavalanche process,xe2x80x9d well known in the photodiode arts.
Semiconductor photodiodes can be constructed from any semiconductor and are typically constructed from single crystal silicon wafers similar to those utilized in the manufacture of integrated circuits. Long-wavelength (e.g., 1310 nm and 1550 nm) photodiodes are typically made from InGaAs with nominally the same lattice constant as Indium Phosphide (InP).
Currently, long-wavelength (about 1200 nm-1650 nm), high-speed photodiodes are all grown on Indium Phosphide (InP) substrates. The integration of receiver photodiode components operating in the long-wavelength range necessitates the utilization of Indium Gallium Arsenide (InGaAs) p-i-n photodetectors and, hence, a technology based on InP substrates in order to avoid strained layers, which cause misfit dislocations due to the lattice mismatch. Unlike Silicon (Si) and Gallium Arsenide (GaAs), however, InP is not as well established and is very fragile. In addition, InP substrates are expensive and not available in sizes as large as GaAs or Si. Finally, gas phase epitaxial growth on InP requires the use of phosphine, which is often not available on particular epitaxial systems and causes several technical problems.
FIG. 1 depicts a cross-sectional view of a prior art monolithic integrated photoreceiver to illustrate prior art photodiode fabrication techniques. As shown in FIG. 1, a photoreceiver 100 may detect and amplify optical beam 50, which impinges on p-i-n photodiode 23 at a wavelength xe2x80xa2s, which can be chosen to be greater than 1.0 xcexcm because, among other reasons, the InP semiconductor may be transparent for those wavelengths. Such an integrated photoreceiver may afford materials compatibility and separate optimization control over the photonics and electronics functions because each device is electrically isolated as well as physically separated from the others. It is important to note, however, that despite materials compatibility and separate optimization control, such a device is relatively expensive, fragile, and difficult to process, requiring complicated integration into components. It should be noted that the semiconductor layers underlying the HBT, while structurally adapted for forming a photodiode, are not electrically or optically active.
In the prior art photoreceiver structure illustrated in FIG. 1, the photodiode and heterostructure bipolar transistor have semiconductor layers selected from the InP/InGaAs material system and grown over a non-patterned Fe doped InP substrate.
Standard fabrication techniques, including metalorganic vapor phase epitaxy (MOVPE), selective wet chemical etching, reactive ion etching and contact metallization, may be utilized to fabricate the prior art illustrated in FIG. 1. These fabrication techniques are well known to those persons of ordinary skill in the art and, thus, are not discussed in detail here.
The prior art device illustrated in FIG. 1 may be epitaxially grown on a planar Fe doped  less than 001 greater than oriented InP substrate 10. The p-i-n epilayers comprise a 4000 xe2x80xa2 thick InP layer 20, a 1.0 xcexcm thick InGaAs layer 21, and a 4000 xe2x80xa2 thick InP layer 22, which may be grown on semi-insulating substrate 10 by MOVPE. Semiconductor layers 20, 21, and 22 thus may form p-i-n photodiode 23.
The epitaxial layers of heterostructure bipolar transistor (HBT) 24 can be grown over the entire structure, such that no intervening processing exists between the deposition of each epilayer. That is, the growth run may be continuous and uninterrupted. HBT 24 may comprise a sub-collector layer 25, collector layer 26, base layer 27, emitter layer 28, and an emitter cap layer 29. It should be noted that HBT 24 may be configured as a single heterostructure device, while photodetector 23 may possess a photoabsorbing layer 21.
After the growth of the photodiode and heterostructure bipolar transistor epitaxial layers, wet chemical etching can be performed to realize the mesa structures thereof. In association with the wet chemical etch, photoresist patterning can be oriented 45 degrees to the [001] and [001] crystallographic direction planes in order to maintain the geometrical shape of the mesa structure. Alternatively stated, the line features of the devices, that is, photodiode 23 and HBT 24, can be delineated so that they are 45 degrees to the [001] and [001] direction planes.
Conventional ohmic contacts, such as AuGe/Au and AuZn/Au, may be deposited on the lateral edges of the mesa in order to provide ohmic contacts 31, 32, 33, and 34 to layer 20, subcollector layer 25, base layer 27, and emitter cap layer 29, respectively. It should be noted that standard metallic deposition techniques can be employed in fabricating the ohmic contact. Additionally, for photodetector 23, an annular alloyed ohmic contact 30 may provide contact to layer 22. Annular ohmic contact 30 may be formed by standard photolithographic, including evaporation and lift-off techniques well known in the art.
To reflect any unabsorbed optical beam 50 back into photoabsorbing region 21 and, thus, increase the quantum efficiency, a non-alloyed Cr/Au metal contact 36 can be deposited in the annular opening of contact 30. After planarization and passivation, such as by using a spun-on polyimide 35, reactive ion etching of the polyimide in an oxygen plasma may be employed to open windows therein in order to facilitate attaching leads, not shown, to the ohmic contacts. After fabrication, substrate 10 may be thinned and polished in order to permit backside illumination.
Based on the foregoing, those skilled in the art can appreciate that prior art manufacturing methods for semiconductor devices, such as photodiodes grown on InP substrates, are expensive, fragile, difficult to process, and additionally difficult to build into components. Such processing complexity also results in inefficiencies and increased manufacturing expenses. In view of reducing the processing complexity, it is, therefore, desirable to develop an alternative technique for forming high-speed photodiodes and devices based on high-speed photodiodes, such as photoreceivers. The present inventors have developed a technique and devices thereof, disclosed herein, which overcome the inefficiencies inherent in the manufacturing of prior art semiconductor devices.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
It is an aspect of the present invention to provide a high-speed photodiode grown on a substrate.
It is another aspect of the present invention to provide methods for growing a high-speed photodiode on a substrate.
It is yet another aspect of the present invention to provide a long wavelength photodiode grown on GaAs.
It is still another aspect of the present invention to provide a metamorphic long-wavelength, high-speed photodiode formed utilizing a superlattice containing substrate and metamorphic epitaxial growth techniques.
The above and other aspects are achieved as is now described. A method and apparatus for fabricating a metamorphic long-wavelength, high-speed photodiode is thus disclosed, wherein a buffer layer matching the substrate lattice constant can be formed at normal growth temperatures and a thin graded region which grades past the desired lattice constant is configured at a low temperature. Upon this graded region, a thin layer can be configured again at low temperatures whose lattice constant is the end lattice constant of the grade and is past the desired lattice constant.
A reverse grade back can then be performed to match the desired lattice constant. Thereafter, a thick layer is formed thereon, based on the desired lattice constant. Annealing can then occur to isolate dislocated material in the graded layer, the reverse graded layer and the layer therebetween. A strained layer superlattice can be then formed thereon, on which a high-speed photodiode can be formed. The strained layer superlattice further isolates the photodiode from the misfit dislocations caused by the metamorphic transition. Note that the terms xe2x80x9cgraded layerxe2x80x9d and xe2x80x9cgrading layerxe2x80x9d are generally synonymous and may be utilized herein interchangeably.
Upon the strained layer superlattice, the photodiode structure can be created. Implant or diffusion layers can be formed based on materials, such as Be, Mg, C, Si, Se, Te, S or Zn. A metal layer can be formed over a cap etch above a P+ region situated directly over an N-active region. The high-speed photodiode can thus be formed on GaAs, which is much cheaper, less fragile and readily configurable into components. Formation of the photodiode can be accomplished utilizing MOCVD as the epitaxial growth technique, although those skilled in the art can appreciate that other growth techniques, such as MBE, can also be utilized in accordance with the methods and apparatus described herein. The preferred substrate orientation is (100) nominally on orientation. Those skilled in the art will appreciate that the term (100) as utilized herein can be expressed as (100),  less than 100 greater than , or [100] or pronounced phonetically as xe2x80x9c1-0-0xe2x80x9d and not to be confused with the reference numeral 100 utilized with regard to FIG. 1.